Big Bang: The powerful explosion of a tiny, very hot object that contained all the matter in the universe. Scientists believe that the big bang was the first thing that happened in the history of the universe, sometime between 12 and 18 billion years ago. Galaxy

galaxy: A huge mass of stars, gas, and dust clouds that exists in one area of space. The Earth and the Solar System are located in the Milky Way galaxy. Scientist think there may be as many as 100 billion galaxies in t...